For a digital camera, the lens receives the light and transmits it to the sensor device, such as Charge Coupled Device, CCD. By the CCD, the collected light is transformed into digital image signals which are further stored in the memory. Typically, the sensor device includes a sensor array, conventional types of which include pixels of “320×240”, “640×480”, “1280×960”, “2048×1536”, and so on.
During the assembling process of a digital image forming device (e.g., a digital camera), it is crucial to align the optical axis of the lens and the center of the sensor array, particularly for those high-end products. “Optical center” recited hereinafter refers to the intersection of the optical axis of the lens and the sensor array. The mis-alignment of the optical center and the center of the sensor array will affect the quality of the outputted digital image.
Conventional alignment within the lens barrel relies on the position pins to mount the sensor array on the image plane. However, for the high-end single lens reflex camera, the lens is replaceable, so the design using the position pin becomes too complicated. U.S. Pat. No. 5,483,284 disclosed that position pins at four corners are used to mount the sensor array on the image plane. However, U.S. Pat. No. 5,483,284 did not disclose how to suitably adjust the position of the sensor array along the direction perpendicular to the optical axis. In addition, U.S. Pat. No. 6,628,339 disclosed a mounting structure including a mounting plate to adjust the position of the sensor array along the directions parallel and perpendicular to the optical axis.
Simply put, for a convergent or divergent optical element, “optical axis” will pass the center of curvature of each surface and also pass through the focal point of the optical element. However, “optical axis” is intangible, so no practical method has existed yet to determine whether the optical center and the center of the sensor array are aligned. As it is known, when light is parallel to the optical axis and is incident to the lens, it is focused and results in the highest illumination at a focal point of the lens. Therefore, the focal point can be determined by detecting the position of the highest illumination, and the alignment of the optical center and the center of the sensor array can be achieved. However, in reality, the noise will make the position of the highest illumination differ from the actual focal point, and hinder the alignment of the optical center and the center of the sensor array.